1. Field of the Invention
The invention relates to the field of computer electronics, and in particular, to a system and method for enabling low-power universal serial bus communications.
2. Related Art
The universal serial bus (USB) protocol is a popular communications protocol that allows a wide range of modern electronic devices and peripherals (e.g., scanners, digital cameras, personal digital assistants, and digital music players) to communicate with another peripheral. The present USB 2.0 specification (“Universal Serial Bus Specification”, Revision 2.0, Apr. 27, 2000) defines three signaling levels that can be supported by USB-compliant devices. The three levels include a low-speed mode operating at 1.5 Mbps at 3.3 V, a full-speed mode operating at 12 Mbps at 3.3 V, and a high-speed mode that signals at 480 Mbps at 400 mV.
Modern high-speed USB 2.0-compliant devices include USB transceivers that are based on a current mode architecture for the high-speed transmitter, in which a current source drives the outgoing USB communications. For example, FIG. 1 shows a high-speed USB transmitter 100 (i.e., a device generating a high-speed USB signal) and a high-speed USB receiver 150 (i.e., a device receiving a high-speed USB signal). A USB cable 140 connects a USB port 111 on USB transmitter 100 to a USB port 151 on USB receiver 150 to enable communication between the two devices.
A USB cable (i.e., USB cable 140) is a four-line serial data bus. Two of the lines are power lines (i.e., VBUS and ground lines), and the other two lines form a pair of differential signal lines (i.e., D+ and D− lines). For clarity, communication between high-speed USB transmitter 100 and high-speed USB receiver 150 will be described with respect to only one half of the differential USB signal (i.e., with respect to signal D+). The inverted signal forming the other half of the differential USB signal (i.e., the D− signal) is generated in a manner substantially similar to that described with respect to the generation of the D+ signal. In accordance with the USB 2.0 specification, USB receiver 150 includes a termination resistor 160 connected between USB port 151 and ground (i.e., the ground supply voltage or lower supply voltage). A D+ signal received at USB port 151 is read from the junction between USB port 151 and resistor 160.
To generate the D+ signal, a high-speed USB transceiver 110 in USB transmitter 100 includes a current source 120, a switch 5105, and a voltage-setting resistor 130. Current source 120, switch 5105, and resistor 130 are connected in series between a supply voltage VDD and ground. When switch S105 is closed, a portion of current I121 supplied by current source 120 flows through resistor 130 to ground (i.e., current I131) to set the output signaling voltage at USB port 111, while another portion of current I121 flows through USB cable 140 and through resistor 160 in USB receiver 150 to ground (i.e., current I161). By switching switch S105 on and off, USB transceiver 110 generates the D+USB signal transmitted from USB transmitter 100 to USB receiver 150 by USB cable 140.
The USB 2.0 specification requires that termination resistor 160 (i.e., the resistor coupled between the USB port and ground in the downstream device) have a resistance value equal to 45Ω±10%. Voltage setting resistor 130 is sized similarly, so that the currents flowing through both resistors are the same (i.e., current I131 is equal to current I161) and that the voltage drops across resistors 130 and 160 are the same. Therefore, to provide the requisite 400 mV D+ signal (i.e., half of the total 800 mV signal specified for high-speed USB 2.0 communications), both currents I131 and I161 must be equal to roughly 8.9 mA (equal to 400 mV (signal amplitude) divided by 45Ω (resistance)). Accordingly, current source 120 must provide a total current I121 equal to roughly 17.8 mA (i.e., the sum of currents I131 and I161).
As the proliferation of USB-compatible devices continues to increase, the importance of power efficiency for those USB devices also increases. Unfortunately, the current-mode architecture used in conventional USB 2.0 transceivers (as shown in FIG. 1) is less than ideal in the realm of power efficiency. Specifically, because resistor 130 in transceiver 110 is used to generate the required signal voltage (i.e., 400 mV), current source 120 must effectively drive two parallel resistance paths to ground (i.e., through resistor 130 in USB transmitter 100 and through resistor 160 in USB receiver 150). Hence, current source 120 must generate twice as much current as is required by USB receiver 150.
Accordingly, it is desirable to provide a system and method for providing high-speed USB communications that reduces power consumption over the conventional current-mode transceiver architecture shown in FIG. 1.